When I replaced the wood bed on my 47 I replaced a shredded wire coming from the top of the gas tank (47 tanks are mounted under the bed) since it was convenient at the time. I left a few feet of wire coiled up under the truck to address later. Well, tomorrow is later. There are no other wires that I see coming from the tank, and nothing going to the gauge. I did buy a 12 v adapter doodad from Classic, but I'm not really sure where it goes. I'm assuming I'll need some power coming from the ignition switch to something, but it's all pretty much a mystery to me. I'm figuring it'll need to be grounded somehow. I'm also assuming that the gauge actually works. I did read thru several posts here but I'd really like to see a schematic sketch of how it all works.
My description pertains to a 49 3100, 47 may be similar. Starting at the tank, the sender is a 0 - 30 ohm variable resistor. The sender is grounded at the tank with one "sender" wire running from the tank to the fuel gauge. The resistance varies depending upon the movement and position of the senders float inside the tank.
The fuel gauge is in effect an ohm meter. The gauges needle deflects from empty to full dependent upon the resistance it reads from the sender.
In order for the gauge to function it needs electrical power and a ground as well as the input from the sender. Generally the gauge grounds through the gauge cluster to the dash.
The gauge has 2 terminals. In my 49 the left terminal (when looking up at it from under the dash) is where the sender wire connects. Mine had a red warning tag on the terminal. The right terminal is where the 6v electrical lead connects. Power is applied to this terminal when the key is turned on.
If your truck has been converted to 12v that doo-dad of which you speak of is probably a resistor that was wired in series with the power lead to the gauge. Generally you remove the power wire from the gauge and connect it to one side of the resistor then connect the other side of the resistor to the power terminal of the gauge. This knocks the power to the gauge down to around 6v - 8v to work the gauge. If your 47 is 6v you don't need the resistor and shouldn't wire it in.
I just converted my 49 to 12V and just went through this exercise.
Dave has given you the hook up. Too simple to need a wiring diagram. Sender wire to the F-ull gauge terminal. Power to the E-mpty gauge terminal. Let us know how it works. Has the sender been replaced recently?
You hope. You might need to assure a good ground by running a ground-wire from a sender-to-tank mounting bolt to a clean ground connection of the frame (I prefer the right frame rail, cause I am anal-retentive (overly cautious}).
OK, thanks guys. Sounds pretty straight forward. I know if the sender was ever replaced it was more than 3 decades ago, probably much longer. It has been converted to 12 volts recently so I'll need the doodad that Dave showed. It's exactly like the one I have.
You and Dave have pointed out the need for a resistor inline with the 12v power-lead into the 6v gauge. As you both probably know, that Runtz-style reducer "resistor" is directional (the in and out sides are one-way).
Hmmm, kinda looks like Dave ran his with the new screw on the doodad to the existing wire and the other end to the gauge. Other than the screw and nut it looks pretty symmetrical.
A couple of things need straightening out in this posting. First, the gas gauge is an electromagnet type device, not even close to an ohm meter. Second, the posting of the pic showing a Runtz reducer is actually a pic of a power resistor. A Runtz is a polarity sensitivity semiconductor, the resistor is not.
I'm gonna hook stuff up and see what happens. I'm thinking my actual odds are slim anyway. Sender is old and probably doesn't work, gauge is old and probably doesn't work, installer is old...
Just Mark, There are some really easy tests you can perform on the fuel indicator system without tearing everything apart. Example, once you have power to the gauge, it can be tested by simply grounding the sender post. Gauge needle will swing one way. Remove the ground wire and it will swing the other. Some other tests require an ohm meter, I would suggest a VOM (the one with a needle). Cheap ones can be had at Harbor Freight, or borrow one from a friend. The guys here will get you through it. You will still be old but wiser.
I wired everything up like I believe it should be. When I first turned the key the needle bopped up to 3/4 tank. About where I thought it should be. Then it moved around a bit and settled on 1/4 tank. I put five gallons if gas in it to fill the tank. The needle did move up to just about 1/2 tank.
Sooo...what's all that mean? Whenever I have something that "kinda works" I usually have a questionable ground somewhere, but I'm by no means knowledgeable with anything electrical.
Most of the time it is either the ground at tank or a bad sender. First add the wire that Tim described at see what happens. Secondly, the sender is a rheostat and an arm slides on a coil. The mechanical wiper contact wears it out. The sensor can also have float failure and internal/electrical corrosion. Chances are that your sender is on it's last leg. If you have to drop the tank to add the wire, then replace the sender also. Your gas gauge will always "kind of work". Be happy with it if it's close.
Thanks. I had the tank out once for cleaning, fresh rubber coupling etc, so I know it'll come it easily. Just not sure I really want to do it. Ugh. Maybe I'll drive it a bit and see if it continues to kinda work. I really just want to know if I'm gonna run out of gas.
I gave up on my fuel gauge and just use an ohmmeter to check fuel level. It is also a good way to check your sender. With a 17.5 gallon tank it works out to .583 ohms per gallon with a 30 ohm sender. I can easily check fuel mileage, and level to 1/10th of a gallon accurately anytime. I thought originally I had a bad sender, but the float was full of fuel was all. I wasted money and bought a new sender and ended up just robbing the float off of it, instead of taking it out and checking it first with a meter. I will probably get a cheap HF ohmmeter, wire it up permanent and stick it somewhere I can see it when needed.
On my 49 GMC project I went with an Autometer 0 - 30 ohm gas gauge and it works dependably well. One day I'll address the OEM gauge cluster but this gauge got the truck back on the road.
